The invention relates to a bath system for the galvanic deposition of metals onto a metal substrate.
Galvanic baths for the deposition of precious metals and precious metal alloys have long been known for use in the field of decorative arts and also in technical fields. The soluble precious metal compounds which are used predominantly are in the form of a cyanide compound (potassium gold cyanide, potassium silver cyanide), a sulfite base (gold sulfite compound), or contain ammonium (palladium compounds). The complexing and stabilizing of the systems select either an excess of the salts builders (cyanide, sulfite), or ammonium as well as the so-called hard complex builders (derivatives of amino-or nitriloacetic acids etc.) or combinations of these.
Systems which contain cyanide are generally known and appreciated for their stability even with complicated and aggravated conditions of use. However, the toxicity of cyanide compounds are considerable sources of danger. Since a considerable interest exists in the use of the dangerous cyanide, continuous efforts and experimentation have been carried out in the area of precious metal baths.
The use of sulfite gold baths and thiosulfate in silver baths has already been described. These systems, however, involve great difficulties with respect to the stability of the bath. Sulfite in complex solutions initiates automatic reduction of precious metals, such as for example gold. Thiosulfate in silver baths on account of the lower complexing strength cannot prevent photochemical reactions of free silver ions. The addition of complex builders to the bath, such as ethylene diamine tetraacetic acid or nitrilotriacetic acid or their derivatives, can indeed slow down the photochemical reaction process but cannot prevent it. Careful control of the working pH level is required, so that a remarkably higher outlay is required with the processing of rinsing and flushing waters and applied baths. The use of a gold sulfite complex is disclosed in German Patent DE-OS-38 05 627, whereby health-endangering pyridine compounds as well as the nerve poison antimony are added to the bath as additional component parts.
All of the bath systems known at this time, and accessible to the expert with knowledge in the art, which were and are operated using replacement materials instead of cyanide, have not come into technical use on account of the difficult handling involved with these baths. Thus WO 92/07975 discloses the use of aminoacetic acid and special glycine as complex builders for palladium and/or silver. However, such a bath solution, which is a costly metal deposition using the so-called potentiostatic deposition method, is not comparable with the traditionally used deposition method referred to as the galvanostatic method. Potentiostatic deposition demands unconditionally a deposition of anolyte and catholyte using a membrane, so that this known method cannot be used for continuous industrial processes. In addition, the use of glycine alone in and of itself with a galvanostatic bath method would also not lead to the desired results.
The invention is directed to aqueous bath systems for galvanic deposition of metals, particularly of precious metals and their alloys, which in accordance with a stable working method are suitable both for decorative and also for technical use. The process of the invention facilitates the application of high quality coatings which remain at the same high quality during the galvanostatic working processes, and where the bath system is free of harmful materials, such as cyanides, sulfites and hard complex builders. Furthermore, the number of salt builders required for complexing of the individual metals is retained as low as possible.
The disadvantages of the prior baths are overcome by the bath system that includes
at least one metal, particularly a precious metal and/or a precious metal alloy in the form of a water-soluble salt, and
at least one water-soluble protein, and/or
at least one water-soluble sulfonic acid, and
at least one water-soluble nitrO-containing material, and
at least one water-soluble surfactant, and
at least one vitamin.